In general, in order to achieve channel power management at the cable disconnection time, an optical repeater output is adjusted or a switch circuit is mounted in a BU (Branching Unit). However, these approaches require complicated circuit configuration and increase cost.
Patent Literature 1 discloses a technique in which a dummy light (ASE light) is disposed in a wavelength channel unallocated band, and the power of the dummy light is controlled at extension time to keep the optical power in the entire band at a constant level irrespective of the number of wavelengths to be multiplexed. Further, Patent Literature 2 discloses a technique in which the optical power of add light to be inserted is adjusted. Patent Literature 2 further discloses, as a third example, a configuration in which a signal level measured using an optical spectrum analyzer installed in a reception terminal station is analyzed so as to adjust the dummy light output.